Induction lamps are electrodeless lamps that typically include a vitreous envelope containing a discharge medium, with the envelope being shaped for operation with an electrical excitation coil. The excitation coil excites the discharge medium to emit light through the induction of electric current in the discharge medium. A principal issue in the design of an induction lamp is the electromagnetic interference (EMI) resulting from the large, high-frequency voltages on the windings of the excitation coil with respect to earth ground. EMI currents flow through stray capacitance between the high voltage windings and earth ground, either directly or via series capacitances including the discharge medium or conductive surfaces employed in the lamp.
One approach to reducing EMI of the foregoing type is to form a conductive coating over the vitreous envelope of an induction lamp, which is then coupled to radio frequency (RF) ground or to some other part of the power supply, or ballast, circuit. While a conductive coating is effective at reducing the noted type of EMI, it requires additional material and manufacturing steps, making the product more costly. It would, therefore, be desirable to provide an induction lamp which, at least in some cases, achieves a tolerable EMI level without the use of a conductive coating on the vitreous envelope of the lamp or other form of shielding. It would be further desirable to provide an additional means of reducing the level of EMI of the noted type in an induction lamp.